I removed a 10 inch limb 9 years ago from a 40 inch diameter silver
maple, taking care not to damage the branch collar. The cut seems to
closing in and healing okay, there is only about a 4 inch hole left and
will be closed over in the next year. The problem is that over the
last few years, there is a hollow area developing behind the cut, it's
about 10 inches back and 18 inches down behind the cut.

I'm concerned the hollow area will weaken the tree or allow disease, if
it hasnt already. The rest of the tree seems fine. I've seen trees
where folks have filled in hollow areas with concrete. Is there
something that should be done to fill in this hollow area, or should it
be left alone.